1. Field
The present disclosure relates to the media access control (MAC) of Ethernet passive optical networks (EPONs). More specifically, the present disclosure relates to MAC control frame extensions.
2. Related Art
In order to keep pace with increasing Internet traffic, network operators have widely deployed optical fibers and optical transmission equipment, substantially increasing the capacity of backbone networks. A corresponding increase in access network capacity is also needed to meet the increasing bandwidth demand of end users for triple play services, including Internet protocol (IP) video, high-speed data, and packet voice. Even with broadband solutions, such as digital subscriber line (DSL) and cable modem (CM), the limited bandwidth offered by current access networks still presents a severe bottleneck in delivering large bandwidth to end users.
Among different competing technologies, passive optical networks (PONs) are one of the best candidates for next-generation access networks. With the large bandwidth of optical fibers, PONs can accommodate broadband voice, data, and video traffic simultaneously. Such integrated service is difficult to provide with DSL or CM technology. Furthermore, PONs can be built with existing protocols, such as Ethernet and Asynchronous Transfer Mode (ATM) protocol, which facilitate interoperability between PONs and other network equipment.
Typically, PONs are used in the “first mile” of the network, which provides connectivity between the service provider's central offices and the premises of the customers. The “first mile” is generally a logical point-to-multipoint network, where a central office serves a number of customers. For example, a PON can adopt a tree topology, wherein one trunk fiber couples the central office to a passive optical splitter/combiner. Through a number of branch fibers, the passive optical splitter/combiner divides and distributes downstream optical signals to customers and combines upstream optical signals from customers (see FIG. 1A). Note that other topologies are also possible, including ring and mesh topologies.
Transmissions within a PON are typically performed between an optical line terminal (OLT) and optical network units (ONUs). The OLT controls channel connection, management, and maintenance, and generally resides in the central office. The OLT provides an interface between the PON and a metro backbone, which can be an external network belonging to, for example, an Internet service provider (ISP) or a local exchange carrier. For EPON, such interface is an Ethernet interface. The ONU terminates the PON and presents the native service interfaces to the end users, and can reside in the customer premises and couples to the customer's network through a customer-premises equipment (CPE).
FIG. 1A illustrates a passive optical network including a central office and a number of customers coupled through optical fibers and a passive optical splitter (prior art). A passive optical splitter 102 and optical fibers couple the customers to a central office 101. Multiple splitters can also be cascaded to provide the desired split ratio and a greater geographical coverage. Passive optical splitter 102 can reside near end-user locations to minimize the initial fiber deployment costs. Central office 101 can couple to an external network 103, such as a metropolitan area network operated by an ISP. Although FIG. 1A illustrates a tree topology, a PON can also be based on other topologies, such as a logical ring or a logical bus. Note that, although in this disclosure many examples are based on EPONs, embodiments of the present invention are not limited to EPONs and can be applied to a variety of PONs, such as ATM PONs (APONs), gigabit PONs (GPONs), and wavelength division multiplexing (WDM) PONs.
FIG. 1B presents a block diagram illustrating the layered structure of a conventional EPON (prior art). The left half of FIG. 1B illustrates the layer structure of an Open System Interconnection (OSI) model including an application layer 110, a presentation layer 112, a session layer 114, a transport layer 116, a network layer 118, a data link layer 120, and a physical layer 122. The right half of FIG. 1B illustrates EPON elements residing in data link layer 120 and physical layer 122. EPON elements include a media access control (MAC) layer 128, a MAC control layer 126, a logic link control (LLC) layer 124, a reconciliation sublayer (RS) 130, medium interface 132, and physical layer (PHY) 134. MAC layer 128 defines a medium independent function responsible for transferring data to and from physical layer 122. In general, MAC layer 128 defines data encapsulation (such as framing, addressing, and error detection) and media access (such as collision detection and deferral process). MAC control layer 126 performs real-time control and manipulation of the operation of MAC layer 128.
In the example of an Ethernet PON (EPON), communications can include downstream traffic and upstream traffic. In the following description, “downstream” refers to the direction from an OLT to one or more ONUs, and “upstream” refers to the direction from an ONU to the OLT. In the downstream direction, because of the broadcast nature of the 1×N passive optical coupler, data packets are broadcast by the OLT to all ONUs and are selectively extracted by their destination ONUs. Moreover, each ONU is assigned one or more Logical Link Identifiers (LLIDs), and a data packet transmitted by the OLT typically specifies an LLID of the destination ONU. In the upstream direction, the ONUs need to share channel capacity and resources, because there is only one link coupling the passive optical coupler to the OLT.
In order to avoid collision of upstream transmissions from different ONUs, ONU transmissions are arbitrated. This arbitration can be achieved by allocating a transmission window (grant) to each ONU. An ONU defers transmission until its grant arrives. A multipoint control protocol (MPCP) specified by the IEEE802.3-2008 and IEEE 802.3av standards can be used to assign transmission time slots to ONUs, and the MPCP in an OLT is responsible for arbitrating upstream transmissions of all ONUs coupled to the same OLT. MPCP uses MAC control messages (frames), such as GATE messages and REPORT messages, to coordinate multipoint-to-point upstream transmission. REPORT messages are used by ONUs to report local queue status to the OLT. GATE messages include discovery GATE messages and normal GATE messages. Discovery GATE messages are used by the OLT during discovery mode to advertise a discovery slot for which all uninitialized ONUs may contend. Normal GATE messages are used by the OLT to grant transmission opportunities to already discovered ONUs.
Although MPCP is able to use MAC control messages to solve the problems of dynamic bandwidth allocation (DBA) for upstream transmissions, many other issues face successful EPON implementation. For example, current flow control mechanisms use a PAUSE message, which is defined by the IEEE 802.3 standard, to signal the other end of the connection to pause transmission for a certain amount of time. Such an approach cannot provide flow control on a per-service (per-queue) basis because the PAUSE frame stops all transmission originating from a physical port. In addition to per-queue flow control, it is desirable to provide other functions using MAC control messages, such as functions that facilitate data encryption, multicasting, laser power control, idle control, etc.